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On 2 December 1942, the first human-made self-sustaining nuclear chain reaction was initiated in CP-1 during an experiment led by Enrico Fermi. The secret development of the reactor was the first major technical achievement for the Manhattan Project, the Allied effort to create nuclear weapons during World War II.
Since nuclear chain reactions may only require natural materials (such as water and uranium, if the uranium has sufficient amounts of 235 U), it was possible to have these chain reactions occur in the distant past when uranium-235 concentrations were higher than today, and where there was the right combination of materials within the Earth's crust.
This is then a self-propagating and thus self-sustaining chain reaction. This is the principle for nuclear reactors and atomic bombs. Demonstration of a self-sustaining nuclear chain reaction was accomplished by Enrico Fermi and others, in the successful operation of Chicago Pile-1, the first artificial nuclear reactor, in late 1942.
The mere fact that an assembly is supercritical does not guarantee that it contains any free neutrons at all. At least one neutron is required to "strike" a chain reaction, and if the spontaneous fission rate is sufficiently low it may take a long time (in 235 U reactors, as long as many minutes) before a chance neutron encounter starts a chain reaction even if the reactor is supercritical.
Although the chain reaction is slower when the supercriticality is low, it still happens in a time so brief that the bullet hardly moves in that time. This could cause a fizzle, a predetonation which would blow the material apart before creating much of an explosion. Thus, it is important that the frequency at which free neutrons occur is kept ...
This is known as a nuclear chain reaction. To control such a nuclear chain reaction, control rods containing neutron poisons and neutron moderators are able to change the portion of neutrons that will go on to cause more fission. [14] Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if monitoring or ...
The nuclear reaction theorised by Meitner and Frisch with the following nuclear chain reaction theorized by Hahn and Strassmann. [1]Nuclear fission was discovered in December 1938 by chemists Otto Hahn and Fritz Strassmann and physicists Lise Meitner and Otto Robert Frisch.
An assembly is critical if each fission event causes, on average, exactly one additional such event in a continual chain. Such a chain is a self-sustaining fission chain reaction. When a uranium-235 (U-235) atom undergoes nuclear fission, it typically releases between one and seven neutrons (with an average of 2.4).